The present invention relates to a color image forming apparatus provided with a plurality of image bearing members and an intermediate transfer belt.
In recent years, image forming apparatuses employing an electro-photographic method, such as a copier, a printer, a facsimile, etc., have been widely introduced in various office environments. Specifically, a lot of color image forming apparatuses capable of outputting a color image onto a sheet have been widely introduced in the market.
Various kinds of configurations have been employed in such the color image forming apparatuses as mentioned in the above. Recently, among other things, a tandem-type color image forming apparatus, in which unicolor toner images are respectively formed on a plurality of photoreceptor drums (image bearing members) and superimposed with each other on the intermediate transfer belt so as to form a color image on it, has been emerged in the market. According to the tandem-type color image forming apparatus, it is possible to achieve both the high image quality and the high productivity in the production of color image products.
In this connection, the intermediate transfer belt, employed in the tandem-type color image forming apparatus, is threaded on a plurality of supporting rollers so as to make it circulatable along the photoreceptor drums. However, since various constituents, such as the plurality of photoreceptor drums, a transfer roller, etc., contact the intermediate transfer belt, sometimes, a part of the intermediate transfer belt slacks under influence of the load incurred onto the intermediate transfer belt during its circulating action. Once the intermediate transfer belt slacks, fluctuations of the circulating velocity of the intermediate transfer belt would occur and result in a transferring deficiency, etc. This would cause a problem of the reproduced image quality.
Conventionally, to cope with the abovementioned problem, various techniques for preventing the intermediate transfer belt from slacking have been proposed so far.
For instance, Patent Document 1 (Tokkai 2001-318507, Japanese Non-Examined Patent Publication) sets forth a technology, which makes it possible to install the tension rollers for giving a tension to the intermediate transfer belt into the inside space surrounded by the inner circumferential surface of the intermediate transfer belt. According to the above technology, it becomes possible not only to alleviate the slack to be generated on the intermediate transfer belt, but also to conduct the image forming operation well.
However, considerations in the technology set forth in Patent Document 1 are not sufficient as a countermeasure for preventing the intermediate transfer belt from slacking, with respect to a color image forming apparatus in which a driving roller for circulating the intermediate transfer belt, a transfer roller for transferring a color toner image formed on the intermediate transfer belt onto a sheet, etc. are separately driven by the separate motors. Referring to FIG. 5 through FIG. 7, the abovementioned point will be detailed in the following.
FIG. 5 shows a brief configuration of a conventional color image forming apparatus.
A conventional color image forming apparatus 1000 is so constituted that unicolor toner images are respectively formed on a plurality of photoreceptor drums 1100, 1101, 1102, 1103, and successively, superimposed with each other onto an intermediate transfer belt 1200 so as to form a full color toner image on it, and then, the full color toner image residing on the intermediate transfer belt 1200 is transferred onto a sheet by a transfer roller 1400.
A driving roller 1300 drives the intermediate transfer belt 1200 to circulate it in a direction indicated by an arrow X shown in FIG. 5 while supporting the intermediate transfer belt 1200. The transfer roller 1400 serves as a roller for transferring the full color toner image formed on the intermediate transfer belt 1200 onto the sheet. A cleaning brush 1500 rotates in a predetermined direction, so as to remove residual toner remaining on the intermediate transfer belt 1200.
The driving roller 1300 is rotated by a driving force transmitted from a driving motor 1301, the transfer roller 1400 is rotated by a driving force transmitted from a driving motor 1401 and the cleaning brush 1500 is rotated by a driving force transmitted from a driving motor 1501. In other words, the driving roller 1300, the transfer roller 1400 and the cleaning brush 1500 are driven by the separate motors, respectively.
FIG. 6 shows an enlarged view of a configuration in the periphery of the transfer roller 1400.
As mentioned in the above, the driving roller 1300 and the transfer roller 1400 are rotated by the driving forces transmitted from the separate driving motors 1301, 1401, respectively. Further, the driving roller 1300 rotates in a direction indicated by an arrow X1 shown in FIG. 6, while the transfer roller 1400 rotates in a direction indicated by an arrow X2 shown in FIG. 6. During the time when the transfer roller 1400 contacts the intermediate transfer belt 1200, since the intermediate transfer belt 1200 is pulled in its circulating direction by the driving force generated by the transfer roller 1400, no slack is generated on the intermediate transfer belt 1200. However, the friction force generated between the transfer roller 1400 and the intermediate transfer belt 1200 is liable to fluctuate under the influences of the image forming conditions, such as a density of an image to be transferred, environmental conditions, presence or absence of a sheet at a transfer nip portion, etc. Sometimes, this would cause the slack on a portion α of the intermediate transfer belt 1200 shown in FIG. 6.
FIG. 7 shows an enlarged view of a configuration in the periphery of the cleaning brush 1500.
The cleaning brush 1500 rotates in a direction indicated by an arrow X3 shown in FIG. 7. Although the cleaning brush 1500 removes the residual toner remaining on the intermediate transfer belt 1200, the friction force generated between the cleaning brush 1500 and the intermediate transfer belt 1200 is liable to fluctuate depending on an amount of residual toner remaining on the intermediate transfer belt 1200. Accordingly, sometimes, this would cause the slacks on portions β1, β2 of the intermediate transfer belt 1200 shown in FIG. 7, under the influence of the driving force for the cleaning brush 1500.